The invention relates generally to elevator controls and, in particular, to a method and an apparatus for preventing local bunching of elevator cars in an elevator group with variable traffic flow.
Hall calls have been allocated to the cars in a group of elevator cars by a large number of different known strategies. The strategy disclosed in the U.S. Pat. No. 4,790,412 determines the estimated time of arrival (ETA) of each elevator car for a specific hall call to be allocated. A count is computed for each car, which count represents the time estimated for the car in question to reach the call floor with the proper service direction to serve the hall call. The hall call assignment is given to the car in the elevator group having the lowest ETA count. This strategy is based on calculating the estimated time of arrival (ETA) to every hall call in the building for each elevator car and then allocating a specific hall call to the car with the lowest ETA.
The concept of car distribution in an elevator system is apparent when the constantly changing patterns of elevator traffic are considered. If the dispatching strategy keeps the elevators well distributed throughout the building, for conditions other than morning up-peak, the cars are in a better position to respond to future hall calls. To accomplish this, some group controls resort to schemes that will spot cars throughout the building when traffic flow has subsided. However, spotting is inefficient. The cars are doing no useful work as they travel to parking floors, therefore wasting energy. The unnecessary wear and tear on the cars increases the maintenance costs.
U.S. Pat. No. 4,790,412 presents a better method for minimizing the bunching of the elevator cars in an ETA dispatching strategy by incorporating a algorithm for solving the distribution problem as part of the assignment algorithm itself. This distribution algorithm improves the distribution by considering previous allocations of the cars when making new allocations.
In the allocation algorithm, the floors of the building are processed sequentially from bottom to top for upward hall calls and from top to bottom for downward hall calls. In the calculation of ETA times for the cars, consideration is given to allocations that have already been made behind the floor that is currently being processed, called the "scan-floor", which is a hall call floor at which the scan has stopped for the purpose of allocating or re-allocating a hall call. This includes stops that a car is committed to make behind the scan-floor due to a car call. If a car is already committed to stops behind the scan-floor, then this car is given a greater chance of getting the allocation for the floor of the hall call being processed by calculating a dynamic bias value Tx and subtracting this bias value from the calculated ETA of the car.
The same procedure applies when a car has an intervening stop between the "present position" and the scan-floor. The "present position" or advanced position floor (avp-floor) is the actual floor location of the car when it is stationary, or it is the floor at which the car can make a normal stop when moving. This calculated dynamic bias Tx will favor the clustering of closely adjacent stops for a given car and thus minimize car bunching. The dynamic bias Tx is inversely proportional to a predetermined travel distance of the elevator car. The predetermined travel distance may be the same regardless of whether the intervening stop is due to a car call or an allocated hall call by using the travel distance between the "present position" of the elevator car being considered for allocation and the scan-floor as shown in the following equation I where K is a selected constant: ##EQU1## The further in terms of travel distance that the scan-floor is from the avp-floor which the car is committed to make to serve its trip list, the smaller is the amount of time subtracted from the ETA of the car. Thus, if the car is a relatively long distance from the scan-floor, a bunching problem is less likely even when it has an intervening stop and the amount of bias reflects this by becoming insignificant.
As an alternative, the predetermined travel distance may depend on whether the intervening stop is due to an allocated hall call or due to a car call. An allocated hall call may be re-allocated, especially if the car is a relatively long way from the hall call floor. Thus, when the intervening stop is due to a hall call, the travel distance from the avp-floor to the scan-floor is used. However, if the intervening stop is due to a car call, which is a stop that the car will have to make, the bias in favor of giving the hall call allocation to the presently considered car may be increased by making the predetermined distance equal to the travel distance from the car call floor to the scan-floor.
The dynamic biasing according to the method shown in the U.S. Pat. No. 4,790,412 prevents a bunching of cars by clustering closely adjacent stops for a given car thereby maintaining a better car distribution throughout the building without placing "dummy" calls for parking floors. Due to this dynamic biasing, the chances are that the cars will already be suitably spaced one from the other as the cars become idle.
Although such a hall call allocation, which allocates closely adjacent stops to a single car, substantially reduces the local bunching of elevator cars, it does however entail a serious disadvantage. As can be seen from the equation I, the amount of biasing Tx is not sensitive to traffic flow levels. The same amount of bias is calculated regardless of the number of calls in the system, i.e. the measure against the local bunching of elevator cars is not re-adjusted to follow the traffic flow level. Although an assignment based on this strategy achieves good distributions with moderate traffic, it often leads to a poor distribution of elevators throughout the building at higher traffic levels. As the traffic flow increases, the elevators start bunching and rely on the randomness of the traffic patterns and the lowering of the traffic flow level to unbunch the cars. During times of high traffic flow level, the result is poor service and an increase in the average waiting time that a passenger has to wait for service. The average waiting time is an industry standard for the measuring of the efficiency of an elevator system.